1. Field of the Invention
This invention relates to a signal sending control system, and more particularly to a signal sending control system for use in a telephone system which employs telephones having a bell for calling and an exchange having a speech network composed of electronic switches.
2. Description of the Prior Art
In the telephone system employing telephones having a bell for calling and an exchange having a speech network composed of electronic switches, which system is now in wide use, since a signal for calling the telephone has an energy level higher than the limit of energy level which can be handled by the electronic switches forming the speech network, it is absolutely necessary to prevent a ringing signal current from flowing in the electronic switches, as well as to prevent noises generated by the connection or disconnection of the ringing signal current from flowing in the electronic switches.
Usually, the ringing signal is intermittently sent to the telephone in such a manner as to be connected to the telephone for 2 seconds at equal intervals of 4 seconds. In the prior art, a ringing signal of, for instance, 20 Hz, is interrupted by a common device so as to be connected for 2 seconds and cut off for 4 seconds, and sent to each telephone. Further, in order to prevent noises, generated by the connection of a ringing signal sending circuit to a subscriber's line or by the disconnection of the former from the latter, from flowing in the speech network, the ringing signal sending circuit is connected to the called subscriber's line for ringing, but disconnected therefrom while no ringing signal is sent out from the ringing signal sending circuit.
With the known ringing signal sending system, the phase of interruption of the ringing signal is limited to only one, as is evident from the above. Accordingly, there are some occasions when no ringing signal is sent to a called telephone immediately after the completion of the terminating connection, resulting in a delay in calling. Further, the prior art system has a disadvantage in that it is incapable of offering a service whereby the kind of a call is indicated by changing the state of interruption of the ringing signal according to, for example, a call from an extension of the private branch exchange or an incoming call from the central office line.
In this kind of exchange, that is, a stored program controlled electronic exchange, control information for controlling the sending and stopping of the ringing signal and the ring back tone is supplied to the line circuits or trunks from the common controller through a signal distributor. In this case, flip-flops are provided to have one to one correspondence to controlled devices of the line circuits or trunks, for example, relays, and the abovesaid control is achieved through these flip-flops. In the prior art, the flip-flop group is provided in a unit larger than that of additional provision of the line circuits or trunks, or basically corresponding to the capacity of the exchange used. The control information transferred in parallel from the common controller is once stored in a buffer register of the signal distributor, and then transferred in parallel to the flip-flop group to bring the flip-flop of a designated row into a state corresponding to the control information in the signal distributor. And, in accordance with the state of the flip-flop, the relay of the line circuit or trunk corresponding to the flip-flop is controlled.
With such prior art systems, since the control information is transferred in parallel from the flip-flops of the signal distributor to the relays to be controlled, a large number of insulated conductors are required for a transmission cable, resulting in an increased cable diameter and hence high manufacturing cost.
Moreover, in the prior art, the flip-flop group is composed of flip-flops whose number is determined in accordance with the maximum number of subscribers or trunks planned to be accommodated in the exchange. Accordingly, if such an exchange is employed in the case of the number of subscribers or trunks being smaller than that of the flip-flops, a certain number of flip-flops are wasted, and in the opposite case, it requires much work to add more flip-flops.